1. Field of the Invention
This invention relates to tension mask color cathode ray picture tubes, and is addressed specifically to improved means and method for the manufacture of tension masks used in such tubes. The invention is applicable to the manufacture of color tubes of various types, including those used in home entertainment type television receivers of both standard resolution and high definition, and in medium-resolution and high-resolution tubes used in color monitors.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the face panel. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. As is well known in the art, the shadow mask acts as a color-selection electrode, or "parallax barrier," which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on assigned phosphor targets. The mask is supported in high tension a predetermined distance from the inner surface of the face panel by one or more support structures known as "rails." The predetermined distance is termed the "Q-height."
The apertured in-process foil that comprises the mask is stretched over the rails and welded thereto. The stretching of a mask is described and claimed in U.S. Pat. No. 4,790,786 to Strauss, of common ownership herewith, and the welding of a mask to the rails is described and claimed in U. S. Pat. No. 4,828,523 to Wichman et al.
A major problem endemic to the manufacture of shadow masks is plugging of the apertures. The problem is particularly acute in the processing of tension mask--the apertures are very small, of the order of 0.0036 inch, and there may be as many as half a million of them in a high-resolution tube. The clogging of even one aperture can be cause for rejection of a tube, and since there are at least 24 distinct steps in the procesing of a shadow mask, occluded apertures are a common problem. Occluding materials include airborne or human borne dust particles, and pieces of lint, which can hook themselves into an aperture and defy most attempts to dislodge them, such as by vibration or air blasting. Even speaking in the vicinity of an in-process mask can result in drops of saliva spraying onto the apertures and occluding them to the detriment of further processing.
The fragility of a foil shadow mask leads to another problem--that of handling it without physical damage. This is especially true of an ultra-thin masks; that is, a mask having a thickness on the order of 0.0006 inch or less. Also, a mask having a pattern of slits is inherently fragile, such as the tied slit mask described and claimed in U.S. Pat. Nos. 4,942,332 and 4,926,089, of common ownership herewith.
2. Prior Art
In U.S. Pat. No. 4,705,357, of common ownership herewith, there is described means for the preventing the occlusion of apertures during the processing of conventional curved shadow masks and flat tension masks. A removable film is disposed on at least one side of the mask to prevent particle occlusion of apertures. The film has an extremely small, substantially uniform thickness dimension, a low index of refraction, and is of such high transparency as to exhibit a negligible optical effect during photoscreening operations. The thickness of the film is in the range of 4 to 6 microinches. The film is also pyrolizable so that it will burn off during an ensuing frit-sealing process. The film is applied to the apertured mask blank by a "flotation" process; that is, the mask blank is immersed in a tank of deionized water, and a filming solution, which may comprise a nitrocellulose compound, is applied to the surface of the water. The water is then drained from the tank. As a result, the film drapes itself over the mask. The final steps in the process include drying of the film and inspection of the mask to detect whether any apertures have been occluded. If so, the film is removed and the process is repeated.
Another example of a very thin film having a thickness in terms of microns is the "pellicle"--which is a form of container for enclosing an in-process semiconductor during the screening process. At the top of the container is an optically clear film spaced from the surface of the semiconductor. Dust particles falling on the film are outside the depth of field of the lens used in the screening process and hence do not affect the process.
A gelatin-based anti-static film may be applied to the back surface of a web of drafting film; its purpose is to prevent static build-up as the web is conveyed at high speeds. The film may be either sprayed or rolled on.